As shown in FIG. 1, a Chinese Utility Model patent with patent No. 200820071126.8 discloses a “big-screen infrared monitor projection objective” for solving focusing problems of projection objective and infrared monitoring objective, which is provided with two focusing devices, i.e., a projection focusing device 17 and a monitoring focusing device 19. Thus, in this patent, the projection objective and infrared monitoring objective need to be focused separately.
Labels in FIG. 1 represent as follows: single negative lens 1, single negative lens 2, single negative lens 3, single positive lens 4, light splitting prism 5, single positive lens 6, single negative lens 7, balsaming lens 8, balsaming lens 9, single positive lens 10, filter 11, single positive lens 12, single negative lens 13, single positive lens 14, balsaming lens 15, projection lens barrel 16, projection focusing hand wheel 17, flange 18, monitoring focusing hand wheel 19, monitoring lens barrel 20, flange 21, CMOS chip 22, chip fixed plate 23, field diaphragm 24.
When the projection distance changes, the projection focusing device 17 needs to be adjusted to make the projection objective to focus, that is to push “the projection lens barrel 16 and the flange 18 to move telescopically in whole”, so as to focus and obtain a clear projection picture. Because the infrared monitoring objective which is mounted on the projection lens barrel 16 moves along at the same time, the focusing device 19 needs to be adjusted to make the infrared monitoring objective to focus, which makes the CMOS chips 22 collect images to meet requirements of interactive operation.
Such type of projection system requires focusing twice, which makes inconvenient in operation; on the other hand, the projection picture is formed by visible light, users can determine whether the focusing is clear by their eyes, but the eyes of users cannot determine whether the requirements of interactive operation is met since the infrared monitoring picture is formed by invisible infrared light.